ALSA: ctxfi: remove unused variable

As of now the pointer to struct dai is not being used anywhere in the
function. So it is safe to remove the variable.
If we are ever doing anything with the container_of(daio, struct dai, daio),
then at that time we can again add the variable.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>